This invention relates to the preparation of 2,5-diketogluconic acid, which is useful as an intermediate for the preparation of ascorbic acid. A solution of 2,5-diketogluconic acid may be selectively reduced to 2-ketogulonic acid, which may be converted to ascorbic acid. The reduction of 2,5-diketogluconic acid may be effected by reduction with an alkali metal borohydride, as disclosed in U.S. Pat. No. 4,159,990 or by a fermentive reduction as described, for example, in U.S. Pat. Nos. 3,922,194, 3,959,076 and 3,963,574.
2,5-Diketogluconic acid is also useful as an intermediate for the preparation of comenic acid by heating in the presence of an acid, as described, for example, in U.S. Pat. No. 3,654,316.
Heretofore, 2,5-diketogluconic acid has been produced by several different varieties of bacteria such as Acetobacter melanogenum, Acetobacter aurantium, Gluconoacetobacter rubiginosus, Gluconoacetobacter liquifaciens and Pseudomonas sesami. The use of these microorganisms, however, is not satisfactory from an industrial point of view because of relatively low yields of 2,5-diketogluconic acid, relatively long fermentation times and because of the production of large amounts of brown or yellow-brown pigments as by-products of cultivation, thereby decreasing the purity of the desired 2,5-diketogluconic acid.
U.S. Pat. No. 3,790,444 relates to the production of 2,5-diketogluconic acid, without accompanying brown pigment, by a new species designated Acetobacter fragum ATCC No. 21409.
U.S. Pat. application Ser. No. 79,668 filed Sept. 28, 1979 now U.S. Pat. No. 4,263,402 relates to the preparation of 2,5-diketogluconic acid in good yields without the formation of pigmented material by the aerobic propagation of Acetobacter cerinus in a glucose containing medium. While total amounts of glucose from about 2.5% to about 20% (wt/vol) can be utilized, it has been found that initial glucose concentrations in the medium greater than about 15% cannot be tolerated by the microorganisms. Accordingly, total amounts of glucose greater than about 15% (wt/vol) can only be utilized by conducting the fermentation at an initial glucose concentration of about 10% to 15% (wt/vol) and thereafter adding further increments of glucose to the fermentation medium during the course of the fermentation, the concentration of glucose in the medium not exceeding about 15% (wt/vol) at any given time. Accordingly, heretofore the overall concentrations, or production capacities, of 2,5-diketogluconic acid have been limited by the relatively low initial glucose concentration that can be employed in the fermentation medium. The productivity of processes employing other microorganisms for the preparation of 2,5-diketogluconic acid, as described hereinabove, is also limited by the necessity of using relatively low initial glucose concentration in the fermentation medium.
It will be readily apparent that a process wherein initial glucose concentrations higher than about 15% (wt/vol), especially levels above 20%, can be tolerated and utilized by the microorganisms for the preparation of 2,5-diketogluconic acid will provide a substantial increase in production capacity and will result in substantial economies of operation. Such a process also avoids the possibility of contamination of the fermentation medium that may occur when production capacities are increased by adding further increments of glucose during the course of the fermentation.